The purpose of this study is to estimate the maximum tolerated dose and dose limiting toxicities of the combination of temozolomide and carmustine (BCNU) in patients with central nervous system (CNS) or systemic tumors refractory to standard therapy. In addition, sampling will be done to characterize the pharmacokinetics of temozolomide and any sequence effects on the pharmacokinetic parameters of temozolomide, and to identify any preliminary evidence of anticancer activity in treated patients.